1. Field of the Invention
The present invention relates to switches generally and more specifically to a magnetically actuated switch (relay) having mercury-wetted, gold-plated contacts that are immersed in a totally fluorinated hydrocarbon.
2. Description of the Prior Art
Many semiconductor-type devices are susceptible to being damaged when subjected to electrostatic discharges of the type which commonly occur when devices are touched. To provide meaningful information as to the susceptability of a particular device, it is important to test the device using a potential waveform which closely approximates that of an electrostatic discharge, namely, one which rises rapidly to a peak potential level and then decays exponentially. For testing, an appropriate potential might have a peak potential level of from 100 to 15,000 volts and an exponential-decay time constant of the order of 200 nanoseconds. Such a waveform would be developed across a resistor if the resistor were coupled by means of an "ideal" switch across a capacitor which had previously been charged to the peak potential level. Unfortunately, real switches are less than ideal.
When employed in the above-mentioned circuit, many prior-art-type switches exhibit what is referred to herein as a "preconduction phenomenon," a phenomenon which precedes by a considerable period of time (in nanoseconds) low ohmic contact. When a switch is closed, before low ohmic contact is made, an arc occurs between the contacts transferring a portion of the energy stored in the capacitor. During this time, due to turbulence caused by the arc, an inconsistent energy transfer occurs which is manifest as peaks, valleys and discontinuities in the potential waveform and which is referred to herein as "chatter." (It is important to contrast the use herein of the term "chatter" with the common use of the term in describing an externally or magnetically induced vibration. Also the use herein of the term "arc" differs from its use in describing the arc which may occur when a switch is opened and the contacts separate, particularly when switching an inductive type load.) Generally, preconduction continues until the capacitor potential falls to what is referred to herein as a "threshold potential." After low ohmic contact is made, contacts of many prior-art-type switches "bounce" for many milliseconds. Many prior-art-type switches also exhibit other undesirable characteristics including "high contact resistance," "corona" (an ionization generally around the contacts), "flashover" (a breakdown due to insufficient contact spacing and/or dielectric strength), "contact heat buildup" and "contact deterioration" (oxidation).
Fluids are commonly employed in prior-art-type switches to reduce corona, to cool the contacts, and to damp contact movement to prevent contact bounce. Unfortunately, many such switches suffer from poor contact resistance; and, many such fluids are flammable and/or toxic. Arc induced decomposition is another problem associated with some of the fluids. Further, most of the fluids provide little dielectric strength improvement; and, such improvement is oftentimes at the expense of increased chatter.
Mercury is also commonly employed in prior-art-type switches. One type of mercury-wetted, vacuum relay employs a moving meniscus of mercury to bridge hard alloy contacts when the contacts are closed. The mercury provides bounceless switching with little, or no, preconduction or chatter. Unfortunately, the relay is position sensitive. Also, the mercury is volatile producing mercury vapor which lowers the dielectric strength of the vacuum. Since the mercury vapor partial pressure increases with temperature, the power handling and duty cycle capacilities of the relay are limited.
The reader may find of interest the following United States patents including the patent of F. Clark (U.S. Pat. No. 1,935,595) which discloses a switch having contacts that are immersed in a tank filled with a fluid that includes a fluoride type halogen (see page 2, lines 123-129). Hoover et al (U.S. Pat. No. 2,850,593) discloses a relay having contacts and a solenoid that are immersed in a fluid (see column 1, lines 34-43). The fluid is purported to provide motion damping to reduce mechanically and magnetically induced vibration-type chatter, to provide better electrical resistance between the contacts, to provide suppression of arcs of the type which occur when the contacts are opened and to conduct heat away from the solenoid coil (see column 3, lines 3-11). E. Hardy (U.S. Pat. No. 2,627,504) discloses a switch having a fluid that has a corrosion prevention feature. A reed switch is disclosed by R. Guichard (U.S. Pat. No. 3,818,392); and, a magnetically actuated switch with a heat-conductive, dielectric fluid is disclosed by B. Baker (U.S. Pat. No. 3,067,279). Zahner et al (U.S. Pat. No. 4,020,306); Lloyd et al (U.S. Pat. No. 4,401,871); J. Gratzmuller (U.S. Pat. No. 3,842,227); Wilson et al (U.S. Pat. No. 3,129,277); and, F. Perrotti (U.S. Pat. No. 1,886,153) disclose fluids including fluorine halogens for use with switches.